The following patents, patent applications, and non-patent literature are believed to reflect the state of the art:
US 2009/0290713 of Belenky;
US 2006/0282660 of Varghese, et al;
US 2006/0098795 of Choti, et al;
US 2005/0254514 of Lynn;
U.S. Pat. No. 7,343,014 to Sovio, et al;
How to Share a Secret, Adi Shamir, Communications of the ACM, Volume 22, Number 11, November 1979, which is available on the Internet at portal.acm.org/citation.cfm?doid=359168.359176; and
A discussion of the pigeonhole principle (also known as Dirichlet's box principle) of mathematics is available on the Internet at en.wikipedia.org/wiki/Pigeonhole_principle.